Time Conversion on Invoices

When invoicing a client i input the hours as 3.50 for 3 and half hours.  But when i go to print invoice it changes it to 3.30 but is still charging as 3.50.  How can i change this setting.  I have customers querying our invoices as 

3.50 x 10.00 = 35.00 but 3.30 x 10.00 = 33.00